Among hoists used for placing and pulling drilling tools and pipe strings, there have been several conventional types, including the planetary clutch type with clutch and brake bands, the disc type clutch with band brakes, and the shoe type clutch, generally with a band brake. These prior art clutches have been satisfactory under proper conditions. However, they have had some disadvantages. Hoists of this sort are generally mounted on vehicles to permit their transport from one drill site to another. They are used in and exposed to all kinds of weather and temperature conditions. Even the enclosed types tend to corrode because of moisture and are difficult to repair in the field. The presence of water causes problems because they are not self-wiping. The enclosed types tend to heat and lose their effectiveness.
One of the objects of this invention is to provide an effective, relatively simple, rugged, easily maintained hoist system, which permits smooth operation and sensitive control.
Another object is to provide such a hoist system with built-in safety factors.
Other objects will become apparent to those skilled in the art in the light of the following description and accompanying drawings.